A blood test will usually reveal a raised muscle enzyme known as creatine kinase (CK). Sometimes a forearm lactate test is performed, although this is not essential.
The diagnosis is confirmed by a muscle biopsy, which shows an excess of glycogen and absence of the muscle enzyme phosphorylase.
In up to 85% of patients from Northern Europe, an abnormality in the gene encoding for muscle phosphorylase, called R50X (previously known as R49X), can be detected on a DNA test (blood test). In practice such testing is rarely helpful and not always available and is not essential to make the diagnosis.